United's supporters at Villa Park initially watched on in horror as the Reds went 2-0 down inside 50 minutes against a side lying 17th in the Barclays Premier League table.
But the 3,127-strong away allocation never gave up hope and roared the Reds on as Javier Hernandez led a thrilling comeback.
The Mexican was involved in all three goals and rightly claimed the post-match plaudits. Sir Alex, however, was at pains to point out the role United's supporters played in the win.
"The supporters were fantastic," he told MUTV. "[Birmingham] is one of the local games for us now, in a way.
"You saw them today - they were great. We were inspired by that. There's no doubt that with great support like that you get inspired and you do something for it.
"The second half today showed we never give in. We can come from behind [and win] against anyone, which is something we've done eight times this season.
"That's encouraging. Anybody who plays against us knows they have to play until the 95th minute."